1. Technical Field
The present invention relates to a persistent volatile memory cell, for memorizing a binary datum during a retention time independent from a supply voltage of the memory cell.
2. Description of the Related Art
Electronic circuits usually comprise two types of memory cells: the volatile memory cells, used for making registers or memory arrays of the type RAM (Random Access Memory), and the non-volatile memory cells, used for making registers or memory arrays of the types ROM (Read Only Memory), EPROM (Electrically Programmable Read Only Memory), EEPROM (Electrically Erasable and Programmable Read Only Memory), Flash, etc.
A datum stored in a volatile memory cell is irremediably lost when the supply voltage of the memory cell disappears. Conversely, a datum stored in a non-volatile memory cell is kept almost indefinitely independently from the supply voltage of the circuit in which the memory cell is implanted.
FIG. 1A shows an embodiment of a volatile memory cell VCELL. The memory cell comprises two inverting gates I1, I2 arranged in closed loop, a writing switch SW and an erase switch SE allowing a datum DTv equal to 1 or to 0 to be stored in the cell. The datum DTv is memorized by the memory cell as long as the inverting gates receive a voltage Vcc, and is erased when the voltage Vcc disappears.
FIG. 1B shows an embodiment of a non-volatile memory cell NVCELL. The cell comprises a floating gate transistor FGT and a read circuit RDCT electrically powered by a voltage Vcc. The memory cell is written by injecting electric charges into the floating gate of the transistor, and is erased by extracting electric charges from the floating gate. The conducting or non-conducting state of the transistor FGT, which depends on the quantity of electric charges trapped in the floating gate, is detected by the read circuit RDCT whose output supplies a datum DTnv. If the supply voltage Vcc disappears, the datum can no longer be read but the state of the memory cell remains the same. The datum DTnv can therefore be read again when the supply voltage reappears.